Conventional ink formulations used in the construction of multilayer circuit structures are typically applied to ceramic substrates and are processed at high temperatures, e.g. 800.degree. C. to 1200.degree. C. Composite substrates have been developed which permit the fabrication of higher power circuits. These composite substrates are typically combinations of metal cores with insulating glass or glass ceramic coatings, e.g. enameled steel or flame sprayed alumina on aluminum. The coatings on such composite substrates tend to delaminate due to differential thermal expansion if the substrates are subjected to the elevated temperatures required for firing conventional resistor inks.
While low firing temperature, i.e. less than 800.degree. C. resistor inks are known, each is deficient in some respect. Air fired low temperature inks typically exhibit poor stability and abrasion resistance, while inert fired low temperature inks may provide acceptable properties but require very costly processing.
What is needed in the art is a low temperature air fired thick film ink for use in the construction of laminar circuit structures on composite substrates.